Chemical and/or biological deposition in industrial fluid processes adversely affects processing efficiency and can adversely impact manufacturing processes, including operational downtime and potentially even plant shutdown. It is understood within the art that mineral and/or biofilm deposition in cooling towers, heat exchangers and other fluid processing vessels reduces critical heat transfer efficiency, decreases flow velocity and can potentially lead to structure fatigue and crack formation. Additionally, the maintenance of service water heat exchanger performance is a safety issue for utility plants, especially nuclear power plants.
The onset of mineral and/or biological deposits in industrial fluid processes has been monitored by measuring either the temperature difference across a heat transfer surface or by measuring physical and chemical changes caused by deposition on a clean surface immersed in the fluid by way of electrochemical, optical, spectroscopic or acoustic methods. Several monitoring systems based on temperature measurement are known and have been used to monitor mineral and biofilm deposition, particularly in heat exchangers and cooling towers. The heat transfer surface in many of these systems is easy to set up and operate. Heat transfer resistance values are provided for the simulated heat transfer surface, which can be correlated to, for example, the overall heat transfer efficiency of heat exchangers. However, deposit monitoring based on temperature measurement is subject to process variations such as changes in process temperature, flow velocity and environmental temperature. For example, changes in power supply to an electric heater in a side stream heat flux simulator can cause errors. Unfortunately, because of the effect of process variables, many commercially available deposit monitoring systems lack the sensitivity required to detect the early onset of deposit accumulation. Consequently, detecting the early onset of deposit accumulation in a cost effective manner has heretofore been difficult to achieve.
Methods used to measure the physical and chemical changes caused by deposition include optical transmittance, fluorescence, and quartz crystal microbalance. The sensitivity of these methods is usually high. However, these methods require relatively expensive instruments. Variations and process parameters affect the measurements, and a heat transfer surface may not be easily incorporated.
In U.S. Pat. No. 4,326,164 a probe for monitoring the corrosion caused by a corrosive medium is provided. The probe comprises a first corrodible resistance element, a second corrodible resistance element having a temperature resistance characteristic similar to that of the first element, each element being in the shape of a rectangular prism, the thickness of the second element being greater than that of the first element.
U.S. Pat. No. 7,077,563 discloses and claims a method for the measurement of differential heat flux, comprising the steps of (a) providing a heat transfer reference surface; (b) providing a heat transfer fouling surface; (c) providing a heat transfer path capable of transferring heat flux between the reference surface and the fouling surface; (d) providing a pair of heat flux sensors, one sensors connected to the reference surface and the other sensor connected to the fouling surface; (e) measuring heat flux values directly from each sensor without having to measure temperature difference between the sensors; (f) calculating differential heat flux data across the heat transfer path from the heat flux values; (g) utilizing the differential heat flux data to detect and quantify deposit accumulation at the fouling surface; and wherein the heat flux values at the reference surface and the fouling surface both change in response to deposit accumulation at the fouling surface.
A disadvantage of the above apparatus and method is the difficulty of establishing a clean heat transfer surface in the same fluid as the heat transfer surface for detection.
Another problem to be overcome is that differential heat flux measurement is subject to flow rate variation. For example, fouling resistance on an active or “in use” detection surface is 5 versus 0 for the clean reference surface. Flow convective heat transfer resistance is 5 for both surfaces. The total heat transfer resistances are 10 for the active detection surface and 5 for the clean surface, a ratio of 2:1. If convective heat transfer resistance changes from 5 to 1 due to flow rate increase, and the total heat transfer resistances are 6 for the detection surface versus 1 for the clean surface, then what results is a ratio of 6:1. With total resistance ratio changing between the two surfaces, the differential heat flux will change, yet not as a result of fouling.
Accordingly, a need exists for an improved system for the monitoring and measurement of deposit accumulation in industrial fluid processes and fluid transport vessels, which is not negatively affected by flow rate. It is desirable to have a fast, accurate and cost-effective system that is able to detect and measure the early onset of chemical and/or biological deposition, while relatively insensitive to process variations such as changes in process temperature, flow velocity and environmental temperature.